Photodetectors responsive to two or more colors have been proposed in the prior art. In some devices the individual color elements are mounted side by side and in some cases one element has a hole in it through which the second element is illuminated. Such devices in effect share the field of view and, when using a common optical system, do not both "see" the same volume of space. In other prior art designs two or more separate detectors are combined into a system that uses dichroic or some other form of beam splitting optics to illuminate the detectors from a common optical input. In still another prior art system a cascade of photodetectors is employed wherein a stack of detectors or possibly a series of photosensitive layers is arranged in a common optical system.
All of the above approaches have some drawbacks. Where separate detectors are used, complicated optics and separate device mountings must be dealt with. Where stacks of devices are used, each element tends to degrade the performance of the others and a sensitivity compromise must be struck.